"Oxygen sensitive" materials, including foods, beverages, and pharmaceutical products, have special packaging requirements to prevent the ingress of exterior oxygen into the package and/or to scavenge oxygen which is present inside the package. In some cases, particularly in the orange juice and brewing industries, oxygen is removed from the product by vacuum or by inert gas sparging, or both. However, it is difficult and expensive to remove the last traces of oxygen by these methods; they have an additional disadvantage of tending to remove volatile product components which are often responsible for some or all of the aroma and flavor of the product.
Molecular oxygen (O.sub.2) can be reduced to a variety of highly reactive intermediate species by the addition of one to four electrons. The carbon--carbon double bonds found in virtually all foods and beverages are particularly susceptible to reaction with these intermediate species. The resulting oxidation products adversely affect the performance, odor or flavor of the product. An "oxygen scavenger" is any material or compound which can remove oxygen from the interior of a closed package either by reacting or combining with the entrapped oxygen, or by promoting an oxidation reaction which yields innocuous products.
Extensive work has been done on incorporating metal-catalyzed oxidizable organic polymers (i.e., polyamides) as oxygen scavengers for the production of plastic containers. However, problems exist with the time/expense required to activate the scavenging composition, the toxicity of the metal-catalysts and the need to prevent interaction of the oxidative reaction byproducts with the package contents and/or environment, and the loss of the oxygen scavenging effect during storage prior to filling.
Other efforts to control oxygen permeation involve the use of high barrier layers which do not scavenge oxygen, but merely retard the transmission of oxygen through the container wall. Of significant commercial success are the five-layer ketchup and hot-fill juice containers developed by Continental PET Technologies, Inc. of Bedford, N.H. These multilayer structures incorporate inner, core and outer layers of PET, and intermediate layers of a high oxygen barrier material such as ethylene vinyl alcohol (EVOH).
There has recently been announced the development of a new family of thermoplastic aliphatic polyketones having improved oxygen barrier properties. See: "Development of a New Polymer Family of Thermoplastic Aliphatic Polyketones," R. L. Danforth and J. M. Machado; "The Chemical Resistance and Barrier Properties of Aliphatic Polyketones," D. H. Weinkauf, P. A. Kinneberg, and C. E. Ash; "Synthesis and Stability of Aliphatic Polyketones," C. E. Ash, D. G. Waters, and A. A. Smaarkdijk; and other papers presented by Shell Development Company, Houston, Tex., at the Society of Plastics Engineers (SPE) 1995 ANTEC Conference, Session W30 (Thermoplastic Materials and Foams). Early polyketones quickly embrittled upon heat aging; however, the recent use of antioxidant additives is said to produce polyketone polymers resistant to thermal oxidative degradation (Ash et al. at page 2320). Also, these new polymers are said to have a low oxygen permeability at room temperature and under dry conditions, resulting from relatively low polymer-penetrant interactions and strong interchain forces that reduce the rate of diffusion (Weinkauf at page 2342). However, as the relative humidity (moisture content) increases, the barrier property is diminished. Therefore, the authors describe their efforts to reduce the negative effect of water on the oxygen barrier property, whereby the polymer is expected to be useful for food packaging applications (Weinkauf at page 2342). Weinkauf et al. also seek to reduce or eliminate aldol condensation which renders the polymer brittle and reduces processability (Weinkauf at page 2342).
The apparent goal of Weinkauf et al. is to: (1) eliminate oxygen degradation through the use of anti-oxidant additives; and (2) reduce oxygen penetration and/or diffusion at higher levels of moisture content, presumably via polymer structure, additives and/or process conditions. Again, this approach seeks to maximize the barrier property, and minimize oxygen reactivity. In a proposed packaging application, it is believed that this polyketone polymer would be highly crystallized in order to enhance the strength (note the polymer's low T.sub.g of 15.degree. C.), which would render the package opaque (nontransparent). Thus, there is no indication that a transparent container can be obtained by this method and clearly, the oxygen reactivity is a property the authors consider undesirable and seek to eliminate.
The variety of oxygen barrier systems and oxygen scavenging systems disclosed in the art is strong evidence of the commercial need for such packaging, and also that the known systems do not satisfy all of the problems. Thus, there is an ongoing need for packaging for oxygen-sensitive products which is both cost-effective and provides a long product shelf life.